1. Field of the Invention
The present invention relates to the field of workholders or hold downs for machinery and specifically to such workholders which act to urge a workpiece, such as a board, against a fence or other guide.
2. Background Information
There are a wide variety of applications in which a substantially linear workpiece is fed into or through a machine which performs some operation on the workpiece. In carrying out this operation, it is common to use a fence, or other guide, to properly align the workpiece. A simple example is ripping a board on table saw. In order for the fence to be effective, the workpiece must be held tightly against the fence. While this may be done by hand, safety, accuracy, and productivity concerns often dictate the use of a workholder.
These workholders may be rollers, springs, featherboards, or any of a variety of similar devices. Their operation is relatively straight forward. A feather board for example is attached to the table of the machine, adjusted so that it contacts the edge of the board at the correct angle and with the desired pressure, and then locked down. Common to most of these workholders is that they must be adjusted to the width of the workpiece and reset if that width changes or another workpiece is substituted. While they can accommodate a small amount of variation in the width of the workpiece, such as an inch or so, they are not intended to accommodate large variation.
This is troublesome in several circumstances. The first is where the edge of the board being contacted by the workholder is irregular. This is common in rough cut lumber for example. The edge may be wavy, varying 2 or 3 inches (or more) along the length of the board. A conventional workholder which is set properly for the narrowest portion, will jamb at the widest portion. If set for the widest portion, it will lose contact with the narrowest portion, failing to perform its function. Even where the variation is within limits, wide variations in pressure on the workpiece may result, affecting feed rates and finish quality of the operation. A second circumstance is where the board tapers, perhaps by design. This is similar to the above, but may have a significantly greater range of width which must be accommodated. A third circumstance is where a series of strips is being removed from the fence side of the workpiece. At each pass, the workpiece becomes narrower, requiring the workholder be reset as often as each pass.
The last situation, above, can be addressed with conventional workholders, but can seriously impact the productivity, and enjoyment, of the operation. The first two situations can not be handled by conventional workholders, and performing the operation without the workholders may pose a serious safety risk by requiring hand guiding.
The above situations may also involve dimensional variation in more than one direction. The simplest is where the workpiece varies in both width and thickness, and the operation requires that the workpiece be held both horizontally against a fence and vertically against the table. Other circumstances may require other orientations of the second workholder.
There is a need for a workholder for use with machinery which can accommodate relatively wide variations in the width of the workpiece, or variations in the contour of the edge which it contacts. It should also readily adapt to width variations between workpieces without requiring manual repositioning. It should maintain a relatively constant pressure against the workpiece despite these variations. Ideally its operation would require minimal intervention by the operator and be adaptable to circumstances or operator preferences. Also ideally, it would also allow for at least two separate workholders, commonly controlled for applying force in two or more directions.